In a high resolution active matrix-type liquid crystal display with a large-sized screen, it is necessary to form picture elements on a scale which is similar to that of a large amount of semiconductor memory such as 1 Megabyte or 4 megabyte DRAM on a screen having a diagonal measuring 10 to 14 inches. It is very difficult to obtain a high yield percentage of finished goods, even in a manufacturing process of good fault tolerance which is, for example, at the same level as that used to produce a DRAM.
In the case of DRAM, auxiliary word and bit lines which serve as auxiliary memory cells are provided so that if there is a fault in a main memory cell, the auxiliary memory cell is used instead of the faulty main memory cell. The DRAM can be used as if it is externally an operationally faultless cell. However, in the case of a liquid crystal display, unless a faulty pixel is repaired at the time of its occurrence, the pixel is never fit for use and, if used, the display will fail to meet acceptable quality standards due to its fault. In an active matrix-type liquid crystal display, for the above reason, it is necessary to provide an auxiliary storage capacitor or non-linear element (TFT, etc.). The larger the active matrix-type liquid crystal display and the higher its resolution, the higher the probability that a fault may occur, and the greater the need for redundant construction.
Generally, in a TFT active matrix-type liquid crystal display, a plurality of scan lines (called gate lines) and a plurality of data lines are formed in the horizontal and the vertical directions, respectively. A TFT, which is a non-linear element, is provided at the respective intersections of the scan lines and the data lines. A respective display electrode, corresponding to each TFT, is provided. Means for applying a scan voltage and a data signal voltage to each scan line and data line, respectively, are also provided. The optical characteristic of a respective liquid crystal element associated with each display electrode is modulated in accordance with the signal voltage. In addition, to prevent picture images from changing due to variations in the characteristics of TFTs or variations in liquid crystal elements, a storage capacitor for each pixel is provided in parallel with the liquid crystal element. There are several methods for forming the storage capacitors.
An example of an arrangement which includes storage capacitors is illustrated with reference to FIG. 1. A data line 32 is disposed in the vertical direction and one electrode of the storage capacitor is connected to a display electrode; the other electrode of the storage capacitor is connected to an electrode line (also called a storage capacitor line or merely a Cs line) 36 formed in the horizontal direction so as to be parallel to a scan line 31.
FIG. 2 is an enlarged view of each pixel, in which an auxiliary storage capacitor is added to achieve a redundant construction. Voltage is applied to a display electrode 34 connected to a source electrode 30 of a TFT 33 selected by the scan line 31 and data line 32 to modulate the optical characteristics of each liquid crystal element. Region 21 represents the semiconductor material of the TFT 33. Regions 24 are formed of an intrinsic amorphous silicon layer and serve as electrical insulators between the scan lines 31 and data lines 32. A storage capacitor 35 is formed and connected in parallel with the display electrode 34 to suppress or mask variation in the characteristics of the liquid crystal elements. One electrode (the top electrode) of storage capacitor 35 is made of the same material as the display electrode 34 and the other is a portion of the Cs line 36, which extends parallel to scan line 31. An insulating film is placed over Cs line 36 to insulate the top electrode of the storage capacitor. The top electrode is an extension of display electrode 34.
In addition, one electrode of an auxiliary storage capacitor 37 is made of the same material as the display electrode 34 and is formed, similarly, over a portion of Cs line 36. Auxiliary storage capacitor 37 is not usually connected to display electrode 34, but is constructed so that it is connected to the display electrode 34, as necessary through a pattern 39, in a switch connection part 38. The pattern 39 is made of the same metal as the scan line 31, with an insulating film between the metal layers.
If the main storage capacitor 35 and the Cs line 36 are short-circuited by a faulty insulating film, etc., that pixel will be displayed as a bright or a dark spot. In this case, the storage capacitor 35 is disconnected from the display electrode 34 along line A--A' by using a YAG laser, or other suitable means. One electrode of auxiliary storage capacitor 37 and the pattern 39, and the display electrode 34 and the pattern 39, are short-circuited, by using the YAG laser, or other suitable means, at a point P and a point Q, respectively, in switch connection part 38, to thereby connect storage capacitor 37 and to repair such a display fault.
As shown in the prior art of FIG. 2, the defect in a pixel, which is caused by a short-circuit between the top electrode of storage capacitor 35 and Cs line 36, can be completely overcome. However, such redundant construction causes the effective area of the liquid crystal element through which light passes to be reduced. In other words, the aperture efficiency, which may be defined as the ratio of the area of one display electrode to the area of one pixel, will decrease.
The aperture efficiency is a major factor in determining the brightness of the display on the screen. It therefore is determinative of the quality of the display and the power consumed by the back light associated with the liquid crystal display.
It is thus a difficult problem to have high yield and a high quality, bright display.
Japanese Published Unexamined Patent Application No.64-73324 discloses the sharing of a Cs line. It does not suggest that its construction is such that an auxiliary storage capacitor is shared by plural as is the case for the present invention, as more fully set forth below.